Hello
I have a question (may be stupid :) ), where I can find a file or something which contains data like mysql database name and and host etc ?
I have openproject installed on server with linux redhat OS.

Not at all… I’m also new to openproject and I found it hard to understand as I’m used to traditional PHP sites.

You may find it at: (if you install it using a package system)
/etc/openproject-ce/conf.d/database
In case is not there, check on: (or the path where you installed it)
/opt/openproject-ce/config/database.yml

If you modify it, don’t forget to restart the service (not sure how is in Redhat, but in Ubuntu is…):
service openproject-ce restart